1. Field of the Invention
The invention relates to a method of normalizing a digital image of an iris of an eye for the purpose of creating an iris code for identification of vertebrates, including humans.
2. Description of Background Art
A method of this type is used in the context of biometric identification systems based on iris analysis.
A biometric personal identification system based on iris analysis has been described by John G. Daugman in U.S. Pat. No. 5,291,560 and in J. G. Daugman: “How Iris Recognition Works”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 14, no. 1, pp. 21-30, January 2004.
Such identification systems take advantage of the fact that the iris of the eye of an individual, which may be a human or a vertebrate, has a characteristic pattern that is unique for that particular individual, so that an iris analysis may be used for uniquely identifying the individual. To that end, an image of an eye of the individual is captured with a digital camera, and image processing algorithms are used for recognizing the pupil and the iris of the eye in the digital image. Then, the iris pattern is normalized so as to compensate the effect of varying dilation or contraction of pupil, and a filter procedure is employed for transforming the normalized image of the iris into a digital code, a so-called iris code, that is unique to the individual and may therefore be used for identification purposes.
Once an iris code of an individual has been created and stored, that individual may be identified by capturing again an image of its eye, creating an iris code on the basis of the new image, and checking the iris code thus obtained against the code that had been stored previously.
In general, identification systems based on iris analysis have to cope with the problem that, when the image of the eye is taken, the iris will normally be distorted to some extent, because the pupil that is surrounded by the iris dilates and contracts dependent on the illumination conditions. It is therefore required for an efficient iris analysis that the image of the iris is normalized in order to eliminate the distortions caused by the pupil dilation and contraction. In known iris analysis methods for individuals that have circular pupils, as most humans, the normalization can be achieved by using polar coordinates and normalizing the radius coordinate to the difference between the radii of the circular outer and inner boundaries of the iris.
US 2009/0208064 A1 describes a pupil segmentation method wherein a pupil region which may have an irregular shape is determined by extracting a region having an intensity that is lower than a certain threshold and then computing the convex hull of that region. This document further describes a method of creating an iris code that is applicable for species, such as horses, which have a horizontally elongated pupil. In this method, the normalization of the iris image and the further procedures for creating the iris code are limited to certain regions of interest in the iris image above and below the pupil.